The invention relates generally to respirators, ventilators and oscillators used to deliver inspiratory gas to a patient. The term xe2x80x9crespiratorxe2x80x9d is used herein to refer to respirators, ventilators and oscillators collectively. Rebreathing circuits, such as so called xe2x80x9ccircle circuitsxe2x80x9d, are used in the operating rooms to conserve volatile anesthetics. There has been a move in the anesthesia field to low bias flow (xe2x80x9cLBFxe2x80x9d) devices as a cost saving measure. While beneficial from an efficiency standpoint, some LBF devices are tedious for the clinician to use because they require manual adjustments of the bias flow to achieve the targeted inspiratory gas flow rate. New anesthesia machines have emerged to facilitate very low bias flows, and provide nearly closed circuit anesthesia. Examples of these LBF anesthesia machines are the Physioflex machine offered by Physio, Inc. and the machine described in U.S. Pat. No. 5,094,235. With these LBF anesthesia machines, the clinician sets the desired oxygen concentration and either the desired inspired or expired anesthetic agent concentration. These LBF anesthesia machines are not designed to allow precise control of the patient""s ventilation using a refined ventilator like those used in intensive care facilities, while at the same time isolating the patient""s breathing gas from that delivered by the ventilator.